Hi dawn68,
the Wave files must be 44.1 kHz / 16-bit / STEREO, because MONO files are not supported.
In case of doubt, please check the Properties of that Wave files, to make shure that they are STEREO.
P.S.
It does not really make sense to convert compressed Audio files (like MP3s) to Wave (uncompressed), because the Audio quality will not be better.
To have a better Audio quality, you have to "rip" a CD directly to Wave format. I use the (relatively old, but good) PC program "Audio Grabber" for this. It can "rip" either to MP3 (several Qualities with fixed or variable bit rate) OR directly to a Wave file.
Best regards,
Chris